Semiconductor substrate, method for manufacturing semiconductor substrate and method for manufacturing semiconductor device

ABSTRACT

To prevent the surface of a base substrate and the bottom surface of a separated semiconductor epitaxial layer from being bonded to each other even after a removal layer is removed, the semiconductor substrate includes a base substrate, a first removal layer provided on the base substrate, a second removal layer provided above the first removal layer, and a semiconductor epitaxial layer provided above the second removal layer, and an etching rate of the second removal layer for a predetermined etching material is larger than the etching rate of the first removal layer for the predetermined etching material.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of InternationalApplication number PCT/JP2019/040926, filed on Oct. 17, 2019, whichclaims priority under 35 U.S.C § 119(a) to Japanese Patent ApplicationNo. 2018-230683, filed on Dec. 10, 2018. The contents of thisapplication are incorporated herein by reference in their entirety.

BACKGROUND

The present invention relates to a semiconductor substrate, a method formanufacturing a semiconductor substrate, and a method for manufacturinga semiconductor device using the semiconductor substrate. There isconventionally a known technique of forming a removal layer and asemiconductor epitaxial layer on a semiconductor substrate andseparating the semiconductor epitaxial layer from the semiconductorsubstrate by removing the removal layer by performing etching (forexample, see Japanese Patent No. 3813123).

In the technique described in Japanese Patent No. 3813123, after theremoval layer and the semiconductor epitaxial layer are formed on a GaAssubstrate serving as a base substrate for growing the semiconductorepitaxial layer, the semiconductor epitaxial layer is separated from theGaAs substrate by removing the removal layer using an etchant such ashydrofluoric acid. When separating the semiconductor epitaxial layerfrom the GaAs substrate in this method, the etching rates of the GaAssubstrate and the semiconductor epitaxial layer are assumed to besignificantly smaller than the etching rate of the removal layer.

When removing the removal layer by performing etching, the surface ofthe GaAs substrate and the bottom surface of the semiconductor epitaxiallayer that are in contact with the removal layer become very flat. As aresult, there has been a problem that the surface of the GaAs substratemight be bonded to the bottom surface of the separated semiconductorepitaxial layer after removing the removal layer by performing etching.If the surface of the GaAs substrate is bonded to the bottom surface ofthe semiconductor epitaxial layer after removing the removal layer byperforming etching, the semiconductor epitaxial layer cannot beseparated from the GaAs substrate. Thus, a technique of preventing thesurface of the GaAs substrate and the bottom surface of the separatedsemiconductor epitaxial layer from being bonded to each other even afterremoving the removal layer is required.

SUMMARY

Therefore, the present invention has been made in view of these points,and an object thereof is to prevent the surface of a base substrate andthe bottom surface of a separated semiconductor epitaxial layer frombeing bonded to each other even after a removal layer is removed.

A semiconductor substrate in the first embodiment of the presentinvention includes a base substrate, a first removal layer that isprovided on the base substrate and composed of AlxGa1-xAs (0.6<x≤0.8), asecond removal layer that is provided above the first removal layer andcomposed of AlyGa1-yAs (0.7≤y<1, y>x), and a semiconductor epitaxiallayer that is provided above the second removal layer and composed ofAltGa1-tAs (0≤t≤1), wherein an etching rate of the second removal layerfor a predetermined etching material is larger than an etching rate ofthe first removal layer for the predetermined etching material.

A method for manufacturing a semiconductor substrate in the secondembodiment of the present invention includes preparing a base substrate,forming a first removal layer composed of Al_(x)Ga_(1-x)As (0.6<x≤0.8)on the base substrate, forming a second removal layer, above the firstremoval layer, that is composed of Al_(y)Ga_(1-y)As (0.7<y≤1, y>x) andhas a larger etching rate for a predetermined etching material than theetching rate of the first removal layer for the predetermined etchingmaterial, forming a semiconductor epitaxial layer composed ofAl_(t)Ga_(1-t)As (0≤t≤1) above the second removal layer, removing thesecond removal layer by performing etching using the predeterminedetching material, and bonding the semiconductor epitaxial layer to adestination substrate different from the base substrate after theremoving.

A method for manufacturing a semiconductor device in the thirdembodiment of the present invention includes preparing a semiconductorsubstrate including a base substrate, a first removal layer that isprovided on the base substrate and composed of Al_(x)Ga_(1-x)As(0.6<x≤0.8), a second removal layer that is provided above the firstremoval layer and is composed of Al_(y)Ga_(1-y)As (0.7<y≤1, y>x), and asemiconductor epitaxial layer that is provided above the second removallayer and composed of Al_(t)Ga_(1-t)As (0≤t≤1), and the semiconductorsubstrate having a larger etching rate of the second removal layer for apredetermined etching material than the etching rate of the firstremoval layer for the predetermined etching material, removing thesecond removal layer by performing etching using the predeterminedetching material, and bonding the semiconductor epitaxial layer to adestination substrate different from the base substrate after theremoving.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a configuration of a semiconductorsubstrate 1 of the present embodiment.

FIG. 2A is a micrograph of the semiconductor substrate 1 used in anexperiment.

FIG. 2B is a schematic view showing an experimental result.

FIG. 3 shows a relation between an etching rate of Al_(z)Ga_(1-z)As anda composition z.

FIG. 4A is for explaining a method for manufacturing a semiconductordevice.

FIG. 4B is for explaining the method for manufacturing the semiconductordevice.

FIG. 4C is for explaining the method for manufacturing the semiconductordevice.

FIG. 4D is for explaining the method for manufacturing the semiconductordevice.

FIG. 5 shows an example in which a support member is provided.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention will be described through exemplaryembodiments of the present invention, but the following exemplaryembodiments do not limit the invention according to the claims, and notall of the combinations of features described in the exemplaryembodiments are necessarily essential to the solution means of theinvention.

<Configuration of a Semiconductor Substrate 1>

FIG. 1 is a schematic view showing a configuration of a semiconductorsubstrate 1 of the present embodiment. FIG. 1 shows a cross-sectionalstructure of a semiconductor epitaxial wafer. The semiconductorsubstrate 1 includes a base substrate 101, a removal layer 102 providedon the base substrate 101, and a semiconductor epitaxial layer 103provided above the removal layer 102.

The base substrate 101 is a base material substrate for crystal-growingthe semiconductor epitaxial layer. A case where the base substrate 101is composed of a group III-V semiconductor is described as an example inthe present embodiment, but the base substrate 101 may be composed of amaterial other than a group III-V semiconductor.

The removal layer 102 is a layer, of which at least a portion is to beremoved by performing etching. The removal layer 102 includes a firstremoval layer 104 provided on the base substrate 101 and a secondremoval layer 105 provided on the first removal layer 104. Although thedetails will be described later, an etching rate of the second removallayer 105 for a predetermined etching material is larger than an etchingrate of the first removal layer 104 for the predetermined etchingmaterial. The predetermined etching material is an etchant or etchinggas having a sufficiently larger etching rate of the second removallayer 105 than an etching rate of the semiconductor epitaxial layer 103.The etching rate of the second removal layer 105 is more than twice theetching rate of the first removal layer 104, for example.

In the following description, the predetermined etching material isreferred to as a used etching material. Examples of the used etchingmaterial include hydrochloric acid, hydrofluoric acid, nitric acid, amixture of hydrochloric acid and nitric acid, organic acid, hydrogenchloride gas, and chlorine gas.

Since the etching rate of the second removal layer 105 for thepredetermined etching material is larger than the etching rate of thefirst removal layer 104 for the used etching material, at least aportion of the first removal layer 104 remains at the time when all ofthe second removal layer 105 is removed by performing etching. At thistime, since the etching of the first removal layer 104 is in progress,the surface of the first removal layer 104, which is exposed at the timewhen the second removal layer 105 is removed by performing etching usingthe used etching material, is rougher than the surface of thesemiconductor epitaxial layer 103 facing the second removal layer 105.Therefore, even if the semiconductor epitaxial layer 103, separated byremoving the second removal layer 105, is in contact with the firstremoval layer 104, the semiconductor epitaxial layer 103 and the firstremoval layer 104 remain separated from each other.

<Composition of Each Layer>

The base substrate 101 is a GaAs substrate, for example. Thesemiconductor epitaxial layer 103 is a semiconductor epitaxial layer tobe removed from the base substrate 101, and composed of Al_(t)Ga_(1-t)As(0≤t≤1), for example. The first removal layer layer 104 is composed ofAl_(x)Ga_(1-x)As (0.6<x≤0.8), and the second removal layer 105 iscomposed of Al_(y)Ga_(1-y)As (0.7<y≤1, y>x), for example. Morepreferably, the first removal layer 104 is Al_(x)Ga_(1-x)As (0.75<x≤0.8)and the second removal layer 105 is Al_(y)Ga_(1-y)As (0.8<y≤1), as arange of material compositions of the first removal layer 104 and thesecond removal layer 105.

Al_(t)Ga_(1-t)As (0≤t≤0.6) is suitable as the material composition of asemiconductor layer constituting the bottom surface of the semiconductorepitaxial layer 103 in contact with the second removal layer 105 (asurface facing the second removal layer 105). It is more preferable thatthe semiconductor layer constituting the bottom surface of thesemiconductor epitaxial layer 103 in contact with the second removallayer 105 is a GaAs layer.

<Thicknesses of the First Removal Layer 104 and the Second Removal Layer105>

The thickness of the first removal layer 104 may be larger than thethickness of the second removal layer 105 so as to make at least aportion of the first removal layer 104 easily remain at the time whenthe second removal layer 105 is removed by performing etching using theused etching material. The thickness of the first removal layer 104 andthe thickness of the second removal layer 105 may be such thicknessesthat at least a portion of the first removal layer 104 remains over theentire region of the first removal layer 104, at the time when thesecond removal layer 105 is removed, even if speeds of etching aredifferent depending on the location of the first removal layer 104.

EXPERIMENTAL EXAMPLE

The semiconductor substrate 1 was produced and etched away in anexperiment. In the semiconductor substrate 1 used in the experiment, thebase substrate 101 was a GaAs substrate. The composition of the firstremoval layer 104 was Al_(x)Ga_(1-x)As (0.75<x≤0.8). The composition ofthe second removal layer 105 was Al_(y)Ga_(1-y)As (0.8<y≤1). Thecomposition of the semiconductor epitaxial layer 103 wasAl_(t)Ga_(1-t)As (0≤t≤0.6).

FIG. 2A is a micrograph of the semiconductor substrate used in theexperiment. FIG. 2B is a schematic view showing the experimental result.The micrograph shown in FIG. 2A is a photograph obtained by imagingusing a Scanning Electron Microscope (SEM) from the arrow direction,while inclining the semiconductor substrate 1 as shown in FIG. 2B.

The photograph shown in FIG. 2A shows a state where the second removallayer 105 is completely etched away by the etchant and a portion of thefirst removal layer 104 is etched away. As shown in FIG. 2A, it can beconfirmed that there is a gap between the semiconductor epitaxial layer103 to be separated from the semiconductor substrate 1 and a remaininglayer 117, which is a layer where a portion of the first removal layer104 remains.

Further, it can be confirmed that an upper surface 115 of the remaininglayer 117 is a rough surface having irregularities whereas a lowersurface 112 of the semiconductor epitaxial layer 103 is flat in FIG. 2A.This is because the etching rate of the first removal layer 104 for theetchant used was smaller than the etching rate of the second removallayer 105, and so only a portion of the first removal layer 104 wasetched away while the second removal layer 105 was completely etchedaway.

After removing the second removal layer 105 by performing etching, theupper surface of the remaining layer 117, a layer in which a portion ofthe first removal layer 104 remained, becomes a rough surface, therebypreventing the semiconductor epitaxial layer 103 from being bonded to asurface on the base substrate 101 side (that is, the remaining layer 117remaining on the base substrate 101) after removing the second removallayer 105 by performing etching. Therefore, the semiconductor epitaxiallayer 103 can be separated from the base substrate 101 properly.

At least a surface roughness Ra of the bottom surface of thesemiconductor epitaxial layer 103 (that is, the surface facing theremoved second removal layer 105) is preferably smaller than a surfaceroughness Rb of the upper surface of the remaining layer 117. Thesurface roughness is an average roughness measured by an Atomic ForceMicroscope (AFM), for example.

According to the experiment by the inventor, after the second removallayer 105 has been etched away until completely being removed, it wasconfirmed that the semiconductor epitaxial layer 103, even when loadingthe semiconductor epitaxial layer 103 in the direction of the basesubstrate 101, can be separated from the base substrate 101 withoutbonding the semiconductor epitaxial layer 103 to the first removal layer104.

FIG. 3 shows a relation between an etching rate of Al_(z)Ga_(1-z)Asconfirmed in an etching experiment in which the inventor usedhydrochloric acid as an etchant for removing the second removal layer105 and a value z that defines the composition of Al_(z)Ga_(1-z)As. Thevertical axis in FIG. 3 is a relative value in a case where the etchingrate is 1 when z=1. It can be confirmed from the result of FIG. 3 that acombination of the first removal layer 104 and the second removal layer105 having a large difference between the etching rate of the firstremoval layer 104 and the etching rate of the second removal layer 105can be selected by setting the composition of the first removal layer104 to be Al_(x)Ga_(1-x)As (0.6<x≤0.8) and the composition of the secondremoval layer 105 to be Al_(y)Ga_(1-y)As (0.7<y≤1, y>x).

<Variation Example of the Configuration of the Semiconductor Substrate1>

In FIG. 1, a structure in which the removal layer 102 includes twolayers, the first removal layer 104 and the second removal layer 105,has been exemplified, but the semiconductor substrate 1 may includeother removal layers in addition to the two layers including the firstremoval layer 104 and the second removal layer 105. At this time, it isdesirable that layers are stacked from the side of the removal layer 102in contact with the base substrate 101 towards the side of the removallayer 102 in contact with the semiconductor epitaxial layer 103 in anorder in which the etching rates become larger.

The semiconductor substrate 1 may further include a third removal layerbetween the first removal layer 104 and the second removal layer 105,for example. In this case, the etching rate of the third removal layerfor the used etching material is larger than the etching rate of thefirst removal layer 104 for the used etching material, and is less thanthe etching rate of the second removal layer 105 for the used etchingmaterial.

In the above description, a case where the base substrate 101 is a GaAssubstrate is exemplified, but the present invention is also applicableto a case where the base substrate 101 is another group III-Vsemiconductor epitaxial substrate. In the semiconductor substrate 1, thebase substrate 101 may be an InP substrate, and the semiconductorepitaxial layer 103 may be a semiconductor epitaxial layer (e.g.,In_(1-x)Ga_(x)As_(y)P_(1-y) (0≤x, y≤1)) that lattice-matches orapproximately lattice-matches on the InP substrate, for example. Here,the removal layer 102 is a semiconductor layer that lattice-matches orapproximately lattice-matches on the InP substrate, and the firstremoval layer 104 is In_(0.52)Al_(0.48)As and the second removal layer105 is In_(0.53)Ga_(0.47)As, for example.

<Method for Manufacturing a Semiconductor Device>

Subsequently, a method for manufacturing a semiconductor device bymoving the semiconductor epitaxial layer 103 to another substrate afterseparating the semiconductor epitaxial layer 103 from the base substrate101 will be described.

FIG. 4 (FIGS. 4A to 4D) is for explaining the method for manufacturing asemiconductor device. First, the semiconductor substrate 1 shown in FIG.1 is prepared. The semiconductor substrate 1 is manufactured by formingthe semiconductor epitaxial layer 103 on the second removal layer 105after forming a stacked structure including the first removal layer 104and the second removal layer 105 on the base substrate 101, for example.The semiconductor epitaxial layer 103 has a semiconductor stackedstructure for forming a predetermined device. The predetermined deviceis any device composed of a semiconductor, such as a light-emittingdiode, a transistor, a sensor, or the like.

It should be noted that, in the present manufacturing method, thesemiconductor device may be manufactured using a manufacturedsemiconductor substrate 1 instead of manufacturing the semiconductorsubstrate 1. That is, a step of preparing the semiconductor substrate 1,in which the first removal layer 104, the second removal layer 105 andthe semiconductor epitaxial layer 103 have already been formed on thebase substrate 101, may be the first step in the method formanufacturing the semiconductor substrate. Further, a step of preparingthe semiconductor substrate 1, in which a predetermined device structureor a portion of the predetermined device structure has already beenformed in the semiconductor epitaxial layer 103, may be the first stepin the method for manufacturing the semiconductor device.

Next, an island where at least a portion of the second removal layer 105to be removed by performing etching (e.g., an outer peripheral surface)is exposed is formed as shown in FIG. 4A. The island shown in FIG. 4A isformed by etching a predetermined area from an outer periphery of thesemiconductor epitaxial layer 103, the first removal layer 104, and thesecond removal layer 105 in the semiconductor substrate 1, for example.

Subsequently, as shown in FIG. 4B, the first removal layer 104 and thesecond removal layer 105 are etched away by a predetermined etchant. Inthe etching step, the etching is continued until the second removallayer 105 is completely removed, and the etching is stopped at the timewhen the second removal layer 105 is removed. In this way, a void 114 isformed by completely removing the second removal layer 105. A partialregion of the first removal layer 104 on the second removal layer 105side (a region 113 in FIG. 4B) is removed, and the remaining layer 117of the first removal layer 104 having a rough surface withirregularities formed on the surface of the first removal layer 104,which is opposite to the surface of the first removal layer 104 incontact with the base substrate 101, is formed. It should be noted thatthe formed irregularities are shown in FIG. 4B in an exaggerated manner.

Subsequently, as shown in FIG. 4C, the semiconductor epitaxial layer 103is separated from the base substrate 101 using a pick-up substrate 121including a pick-up bump 122, for example. The pick-up bump 122 is anorganic material layer and can be formed by a standard photolithographyprocess by applying a photosensitive organic material on a semiconductorsubstrate, for example. The organic material layer may be formed byapplying an organic material on a pick-up base substrate serving as abase of the pick-up substrate 121 using a spin-coating method, a dipmethod, or the like, or may be formed by attaching an organic materialfilm on the pick-up base substrate, for example.

Subsequently, as shown in FIG. 4D, the semiconductor epitaxial layer 103separated from the base substrate 101 by the pick-up substrate 121 isbonded onto a destination substrate 130 by pressure. A surface treatmentprocess, such as a bonding surface activation process, may be performedprior to the bonding. Further, as a material layer different from thematerial of the destination substrate 130, another semiconductor layer,a metal layer, or an insulating film layer may be provided on thedestination substrate 130, for example.

The pick-up substrate 121 is separated after bonding the semiconductorepitaxial layer 103 onto the destination substrate 130 (not shown).Manufacturing of the semiconductor device is completed by appropriatelyforming an interlayer insulating film, a metal wiring film, or the likeon the semiconductor epitaxial layer 103 after separating the pick-upsubstrate 121.

Variation Example 1

After forming an island including the semiconductor epitaxial layer 103,the first removal layer 104, and the second removal layer 105 on thebase substrate 101 as shown in FIG. 4A, a support member may be providedto support the semiconductor epitaxial layer 103 such that thesemiconductor epitaxial layer 103 is positioned above the base substrate101.

FIG. 5 shows an example in which a support member 106 is provided. Thesupport member 106 is a semiconductor thin film, an organic thin film,or an inorganic thin film that couples the semiconductor epitaxial layer103 and the base substrate 101, for example. The support member 106 isformed such that a partial region of the support member 106 is incontact with side surfaces of the first removal layer 104 and the secondremoval layer 105, for example. A portion of the support member 106 maybe formed such that the support member 106 extends to the top surface ofthe semiconductor epitaxial layer 103 or the surface where the basesubstrate 101 is exposed. The thin film constituting the support member106 has a sufficiently smaller etching rate for the etching materialused in removing the second removal layer 105 than the etching rate ofthe second removal layer 105, and remains in a state where thesemiconductor epitaxial layer 103 and the base substrate 101 are coupledto each other at the time when the second removal layer 105 is removed.

The support member 106 may be formed in the following manner, forexample. When forming the support member 106 from a semiconductor thinfilm such as amorphous Si or poly-Si, the semiconductor thin film can beformed by appropriately selecting a sputtering method, a ThermalChemical Vapor Deposition (TCVD) method, a Plasma Chemical VaporDeposition (PCVD) method, or a Low Pressure Chemical Vapor Deposition(LPCVD) method, for example. In this case, after forming thesemiconductor thin film, a structure of the support member 106 can beformed by dry etching such as photolithography and a Reactive IonEtching method (RIE), or wet etching using a predetermined etchant.

When forming the support member 106 from an inorganic thin film, theinorganic thin film can be formed by a sputtering method, a thermal CVDmethod or a PCVD method, for example. After forming the inorganic thinfilm, the structure of the support member 106 can be formed by dryetching such as photolithography and an RIE method or wet etching usinga predetermined etchant.

When forming the support member 106 from an organic material thin film,the support member 106 can be formed by applying or attaching aphotosensitive organic material, and then forming a predeterminedstructure by photolithography, followed by performing a bakingtreatment, for example. For the etchant used in forming the structure ofthe support member 106, it is desirable to select an etchant or etchinggas having smaller etching rates of the first removal layer, the secondremoval layer and the semiconductor epitaxial layer 103 compared to theetching rate of the support member 106.

In this way, providing the support member 106 in a step of removing thesecond removal layer 105 makes it possible to reduce the possibilitythat the semiconductor epitaxial layer 103 comes into contact with thefirst removal layer 104 or the base substrate 101. In particular,providing the first removal layer 104 having a smaller etching rate thanthe second removal layer 105 makes it possible to prevent thesemiconductor epitaxial layer 103 from being bonded to the basesubstrate 101 even if the thin film constituting the support member 106is damaged or does not remain.

It should be noted that the support member 106 is preferably formed suchthat the support member 106 is easily cut off when a force is applied ina direction in which the semiconductor epitaxial layer 103 moves awayfrom the base substrate 101. The support member 106 has a shape (e.g., agroove) in which the thickness of a portion of the region in contactwith the side surface of the second removal layer 105 is smaller thanthe thickness of other regions, and so the support member 106 can beeasily cut off after the second removal layer 105 is removed, forexample. In this case, a step of cutting off the support member 106 bypulling up the semiconductor epitaxial layer 103 may further be includedbetween a removing step of removing the second removal layer 105 byperforming etching using a predetermined material and a bonding step ofbonding the semiconductor epitaxial layer 103 to the destinationsubstrate 130 that is different from the base substrate 101.

The support member 106 whose thickness of a predetermined region issmaller than the thickness of the other regions can be manufactured bythe following method, for example. First, regions other than a regionwhose thickness is to be reduced (for example, a region where a grooveis formed), is covered with a resist. Subsequently, after performingetching such that the thickness of the support member 106 becomes apredetermined thickness by the wet etching using an etchant for etchingthe material of the support member 106 or the dry etching such as RIE,the resist is removed.

Variation Example 2

In the above description, a case where the etching rate of the secondremoval layer 105 for the predetermined etching material described aboveis larger than the etching rate of the first removal layer 104 has beenexemplified, but the etching rate of the first removal layer 104 may beequal to or larger than the etching rate of the second removal layer105. In this case, however, the thickness of the first removal layer 104is required to be larger than the thickness of the second removal layer105 so that at least a portion of the first removal layer 104 remainswhen the removal of the second removal layer 105 by performing etchingis completed.

<Effects of the Semiconductor Substrate 1>

The semiconductor substrate 1 of the present invention includes at leasttwo removal layers having different etching rates between thesemiconductor epitaxial layer 103 to be separated from the basesubstrate 101 and the base substrate 101. Therefore, it is possible toprevent the semiconductor epitaxial layer 103 from being bonded to thebase substrate 101 after the semiconductor epitaxial layer 103 isseparated.

Further, by manufacturing a semiconductor device using the semiconductorsubstrate 1, the semiconductor epitaxial layer 103 having a flat surfaceis easily moved to the destination substrate 130, and the semiconductorepitaxial layer 103 that has been moved is then processed, and thus itis possible to manufacture various semiconductor devices.

The present invention is explained on the basis of the exemplaryembodiments. The technical scope of the present invention is not limitedto the scope explained in the above embodiments and it is possible tomake various changes and modifications within the scope of theinvention. For example, the specific embodiments of the distribution andintegration of the apparatus are not limited to the above embodiments,all or part thereof, can be configured with any unit which isfunctionally or physically dispersed or integrated. Further, newexemplary embodiments generated by arbitrary combinations of them areincluded in the exemplary embodiments of the present invention. Further,effects of the new exemplary embodiments brought by the combinationsalso have the effects of the original exemplary embodiments.

What is claimed is:
 1. A semiconductor substrate comprising: a basesubstrate; a first removal layer that is provided on the base substrateand composed of Al_(x)Ga_(1-x)As (0.6<x≤0.8); a second removal layerthat is provided above the first removal layer and composed ofAl_(y)Ga_(1-y)As (0.7≤y≤1, y>x); and a semiconductor epitaxial layerthat is provided above the second removal layer and composed ofAl_(t)Ga_(1-t)As (0≤t≤1), wherein an etching rate of the second removallayer for a predetermined etching material is larger than an etchingrate of the first removal layer for the predetermined etching material.2. The semiconductor substrate according to claim 1, wherein the firstremoval layer is composed of Al_(x)Ga_(1-x)As (0.75<x≤0.8).
 3. Thesemiconductor substrate according to claim 1, wherein the second removallayer is composed of Al_(y)Ga_(1-y)As (0.8<y≤1).
 4. The semiconductorsubstrate according to claim 1, wherein the thickness of the firstremoval layer is larger than the thickness of the second removal layer.5. The semiconductor substrate according to claim 1, wherein the surfaceof the first removal layer that is exposed at the time when the secondremoval layer is removed by performing etching using the predeterminedetching material is rougher than the surface of the semiconductorepitaxial layer facing the second removal layer.
 6. The semiconductorsubstrate according to claim 5, wherein the semiconductor epitaxiallayer is composed of Al_(t)Ga_(1-t)As (0≤t≤0.6).
 7. The semiconductorsubstrate according to claim 1, further comprising: a third removallayer provided between the first removal layer and the second removallayer, wherein an etching rate of the third removal layer for thepredetermined etching material is larger than the etching rate of thefirst removal layer for the predetermined etching material, and smallerthan the etching rate of the second removal layer for the predeterminedetching material.
 8. The semiconductor substrate according to claim 1,further comprising: a support member for supporting the semiconductorepitaxial layer such that the semiconductor epitaxial layer ispositioned above the base substrate, wherein the support member is incontact with the base substrate, the first removal layer, the secondremoval layer, and the semiconductor epitaxial layer, and has a shape inwhich the thickness of a portion of a region in contact with the secondremoval layer is smaller than the thickness of other regions.
 9. Amethod for manufacturing a semiconductor substrate comprising the stepsof: preparing a base substrate; forming a first removal layer composedof Al_(x)Ga_(1-x)As (0.6<x≤0.8) on the base substrate; forming a secondremoval layer, above the first removal layer, that is composed ofAl_(y)Ga_(1-y)As (0.7<y≤1, y>x) and has a larger etching rate for apredetermined etching material than the etching rate of the firstremoval layer for the predetermined etching material; forming asemiconductor epitaxial layer composed of Al_(t)Ga_(1-t)As (0≤t≤1) abovethe second removal layer; removing the second removal layer byperforming etching using the predetermined etching material; and bondingthe semiconductor epitaxial layer to a destination substrate differentfrom the base substrate after removing the second removal layer.
 10. Themethod for manufacturing a semiconductor substrate according to claim 9,further comprising, between the forming the semiconductor epitaxiallayer and the removing the second removal layer, a step of: providing asupport member for supporting the semiconductor epitaxial layer suchthat the semiconductor epitaxial layer is positioned above the basesubstrate.
 11. The method for manufacturing a semiconductor substrateaccording to claim 10, further comprising, between the removing and thebonding, a step of: cutting off the support member.
 12. A method formanufacturing a semiconductor device comprising the steps of: preparinga semiconductor substrate including a base substrate, a first removallayer that is provided on the base substrate and composed ofAl_(x)Ga_(1-x)As (0.6<x≤0.8), a second removal layer that is providedabove the first removal layer and is composed of Al_(y)Ga_(1-y)As(0.7<y≤1, y>x), and a semiconductor epitaxial layer that is providedabove the second removal layer and composed of Al_(t)Ga_(1-t)As (0≤t≤1),and the semiconductor substrate having a larger etching rate of thesecond removal layer for a predetermined etching material than theetching rate of the first removal layer for the predetermined etchingmaterial; removing the second removal layer by performing etching usingthe predetermined etching material; and bonding the semiconductorepitaxial layer to a destination substrate different from the basesubstrate after removing the second removal layer.